In a broad sense, this invention relates to methods for enhancing cooling efficiency in high temperature components. In some of the more specific embodiments, the invention is directed to providing roughness on the internal surfaces of cooling holes within turbine engine components.
A number of techniques are currently available for maintaining the temperature of turbine engine components below critical levels. As an example, coolant air from the engine compressor is often directed through the component, along one or more component surfaces. Moreover, relatively long radial cooling holes are often drilled through turbine blades, to serve as conduits for coolant air.
The radial cooling holes are often formed by a process known as Shaped Tube Electrolytic Machining, or “STEM drilling”. The STEM process is an electrochemical machining technique which is especially useful for drilling small holes with large depth-to-diameter ratios. A very important advantage of this process is that it can be used to provide roughness to the inner surface of the cooling holes. The roughness greatly enhances heat transfer through the holes. STEM drilling is mentioned in various references, such as U.S. Pat. Nos. 5,927,946 and 5,820,744.
In brief, STEM systems often utilize one or more negatively-charged titanium tubes, an acid electrolyte, and a positively-charged substrate or work-piece. The electrolyte is pumped into the substrate, dissolving the metal in the pre-selected path of the cooling hole. In order to produce roughness within the holes, the injection of the electrolyte is intermittently stopped as the depth of the hole is being increased. This intermittent action results in the formation of protrusions along the length of the cooling holes. The protrusions provide the roughness and surface area required for the enhanced heat transfer.
While STEM drilling is a useful technique in many instances, it has some disadvantages also. For example, the process is very slow. Thus, when a significant number of holes have to be drilled, the considerable time required can result in high processing cost. The equipment required can also be quite expensive. Moreover, STEM drilling can produce etching debris which can decrease heat transfer efficiency in the hole, if not removed properly. Furthermore, STEM drilling sometimes results in inconsistent roughness patterns on the passage hole surface, which also can adversely affect heat transfer efficiency.
Thus, new methods for providing turbulation to the surface of passage holes in an article would be welcome in the art. The methods should be capable of securing the turbulation to any selected area of the hole wall. Moreover, the methods should allow one to change the shape, size, and pattern of the desired turbulation, as well as its composition. The methods should also be compatible with any other processes being used in conjunction with the article, and should not add excessive costs to the fabrication thereof.